Passive radio frequency identification (RFID) tags, as they themselves are not designed with any battery, operate relying on electromagnetic energy sent from a card reader. Due to their simple structure and economical practicality, passive RFID tags have been widely applied in the fields of logistics management, asset tracking and mobile healthcare.
When a passive RFID tag operates, it will absorb electromagnetic energy, sent from a card reader, from the surrounding. After absorbing the energy, the passive RFID tag rectifies part of the energy into DC power for powering internal circuits of the passive RFID tag; and the passive RFID tag further inputs the other part of the energy to an internal modulation/demodulation circuit which will demodulate an amplitude modulation signal carried in this energy and send the demodulated signal to a digital baseband portion of the passive RFID tag for processing.
As the distance between the passive RFID tag and the card reader varies, the electromagnetic energy absorbed by the passive RFID tag during operating from the surrounding varies too. When the passive RFID tag is too close to the card reader or the electromagnetic energy sent from the card reader is too high, the strength of a signal received by the passive RFID tag will also be high, so that the voltage sensed on the coil exceeds the voltage-withstanding limit of a transistor for the rectifier module in the chip. As a result, the transistor is damaged permanently, and the RFID tag no longer functions.
The passive RFID tag transmits data to the card reader in a load modulation manner, and the coil at the card reader side acquires the data upon detecting change in the impedance of the coil at the RFID tag side. When the passive RFID tag is too close to the card reader or the electromagnetic energy sent from the card reader is too high, a load modulation signal coupled from the RFID tag side is likely to result in saturation of the receiving end of the card reader, thus to fail the communication. Such failure is more likely to occur in an RTF (Reader Talk First) communication mode where the card reader sends a command first and then waits for a response from the RFID tag.
Meanwhile, in the RTF (Reader Talk First) communication mode, the card reader sends energy to the tag first, which is called downlink communication; and the tag, after coupling this energy and reading a demodulation signal in the energy, executes the demodulation of the command and sends results of processing back to the card reader, which is called uplink communication. In the event of half-duplex communication, during the uplink communication, since the card reader has stopped sending energy to the tag, the demodulation of the command and the uplink communication by the tag both rely on the limited energy sent from the card reader during its downlink communication. If the energy is too low, the tag can not demodulate the command well and execute the uplink communication, that is, the card reader can not read data in the tag. Therefore, it is necessary to place the card reader closer to the tag in order to send more energy to the tag in the downlink communication stage, so that the tag can complete the whole command demodulation and the uplink communication. In short, when the energy in the tag is too low, the read-write distance of the tag will be significantly influenced, and hence, it is required to perform effective control management to the power supply terminal of the tag. When the energy of the tag is too high, it is required to open the discharge path to discharge the surplus charge. When the voltage of the tag side is too low, it is required to close all discharge paths to realize the most effective use of power.
In order to solve the aforementioned problems of voltage-withstanding reliability, saturation of reception at the card reader, and energy retention during the uplink communication of the tag, it is required to provide an amplitude limitation processing circuit in the interior of an RFID tag chip circuit, in order to ensure that the voltage across both ends of an antenna on the RFID tag is limited to a predetermined value.